Optical coupling using polarization beam displacer

ABSTRACT

An optical coupling apparatus for coupling an optical fiber to a photonic chip is described. The apparatus includes a collimating microlens for collimating light from the optical fiber; a polarization splitting beam displacer for separating the light collimated by the collimating microlens into orthogonally polarized X and Y component beams; at least one focusing microlens for directing the X and Y component beams separately onto the photonic chip; and first and second surface grating couplers (SGCs) orthogonally disposed on the photonic chip and configured for operation in a same polarization state, for coupling the X and Y component beams, respectively, to the photonic chip.

REFERENCE TO RELATED APPLICATIONS

The present application claims priority from U.S. Provisional PatentApplication No. 62/184,699 filed on Jun. 25, 2015, and U.S. patentapplication Ser. No. 15/162,765 filed on May 24, 2016, both of which areincorporated herein by reference in their entireties.

TECHNICAL FIELD

The present description relates to optical coupling, and moreparticularly, to an optical coupling apparatus incorporating apolarization beam displacer.

BACKGROUND

For many photonic applications such as high-capacity photonic switching,photonic transmitters and photonic receivers, it is necessary to couplelight between optical fibers and a photonic chip. There are sizelimitations to the design of the coupler. A typical large size photonicchip may be in the scale of 25 mm by 25 mm. Single mode optical fibersproduced commercially typically have a cladding diameter of 125 nm to250 nm. In such a case, a maximum of 100-200 fibers can be arranged in aline across a photonic chip. To achieve a larger number count, such as1000 fibers, a two-dimensional array of fibers may be required. Couplingan array of fibers to a photonic chip can be achieved using an array ofoptical couplers.

Light propagating in most optical fibers contains two orthogonalpolarization components. Photonic chip circuits, however, often operatein a single polarization. One solution is to implement so calledpolarization diversity to perform the desired operations of the chip ontwo polarization components in parallel. For example, in a photonicswitch with polarization diversity, the switching between inputs andoutputs may be done by switching both components in an identical manner.As well, in a receiver for a polarization encoded signal, each componentmay need to be directed to a separate receiving circuit operating in asingle polarization of light.

Accordingly, an apparatus is desired that can couple light between afiber array and a photonic chip with polarization diversity.

SUMMARY

The following presents a summary of some aspects or embodiments of thedisclosure in order to provide a basic understanding of the disclosure.This summary is not an extensive overview of the disclosure. It is notintended to identify key or critical elements of the disclosure or todelineate the scope of the disclosure. Its sole purpose is to presentsome embodiments of the disclosure in a simplified form as a prelude tothe more detailed description that is presented later.

Described herein are apparatuses and methods for coupling light betweenan optical fiber (or an optical fiber array) and a photonic chip. Inaccordance with disclosed embodiments, the optical coupling apparatususes an off-chip polarization splitter/combiner and on-chip surfacegrating couplers (SGCs) configured for operation in a singlepolarization state.

In accordance with one aspect of the disclosure, there is provided anoptical coupling apparatus for coupling an optical fiber to a photonicchip. The apparatus includes a collimating microlens for collimatinglight from the optical fiber; a polarization splitting beam displacerfor separating the light collimated by the collimating microlens intoorthogonally polarized X and Y component beams; at least one focusingmicrolens for directing the X and Y component beams separately onto thephotonic chip; and first and second surface grating couplers (SGCs)orthogonally disposed on the photonic chip and configured for operationin a same polarization state, for coupling the X and Y component beams,respectively, to the photonic chip.

In accordance with some embodiments, the at least one focusing microlenscomprises a single focusing microlens for directing both the X and Ycomponent beams, respectively, onto the first and second SGCs. Inaccordance with other embodiments, the at least one focusing microlenscomprises first and second focusing microlenses for directing the X andY component beams, respectively, onto the first and second SGCs,respectively.

In accordance with some embodiments, the optical coupling apparatusfurther comprises an array of optical fibers including the opticalfiber; a collimating microlens array including the collimatingmicrolens, for collimating light from the array of optical fibers, afocusing microlens array including the at least one focusing microlens,for directing the X and Y component beams separately onto the photonicchip; and an SGC array configured for operation in a same polarizationstate and comprising first and second SGC sub-arrays including the firstand second SGCs, respectively. The polarization splitting beam displaceris configured for separating light collimated by each microlens of thecollimating microlens array into orthogonally polarized X and Ycomponent beams; and the first and second SGC sub-arrays are configuredfor coupling the X and Y component beams, respectively, separately tothe photonic chip.

In accordance with another aspect of the disclosure, there is provided amethod for coupling light between an optical fiber and a photonic chip.The method comprises collimating light from the optical fiber;separating the collimated light into orthogonally polarized X and Ycomponent beams; directing the X and Y component beams separately ontothe photonic chip; and coupling the X and Y component beams to thephotonic chip respectively by first and second surface grating couplers(SGCs) orthogonally disposed on the photonic chip and configured foroperation in a same polarization state.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure will become more apparentfrom the description in which reference is made to the followingappended drawings.

FIG. 1 is a perspective view of an optical coupling apparatus, accordingto one embodiment of the disclosure.

FIG. 2 is an illustration of the divergence of the X, Y component beamsof the embodiment of FIG. 1.

FIG. 3 is a perspective view of an optical coupling apparatus, accordingto another embodiment of the disclosure.

FIG. 4 is an illustration of the divergence of the X, Y component beamsof the embodiment of FIG. 3.

FIG. 5 is a perspective view of the embodiment of FIG. 3 including atwo-dimensional fiber array.

FIG. 6 is an example of a two-dimensional fiber array that can be usedas part of or in connection with the optical coupling apparatus.

FIG. 7 is a cross-section view of a collimating microlens array,according to one embodiment.

FIG. 8 is a cross-section view of a focusing microlens array, accordingto one embodiment.

FIG. 9 is an array of surface grating couplers on a photonic circuitwith polarization diversity, according to one embodiment.

FIG. 10 is a close-up view of two surface grating couplers in theembodiment of FIG. 9.

FIG. 11 is an example of intersections of X and Y component beams ontheir corresponding focusing microlenses.

FIG. 12 is an elevation view of the optical coupling apparatus,according to one embodiment.

FIG. 13 is an elevation view of the optical coupling apparatus,according to another embodiment.

FIG. 14 is an optical coupling apparatus with image magnifying,according to one embodiment.

FIG. 15 is an optical coupling apparatus with image magnifying,according to another embodiment, illustrating a clipping effect.

FIG. 16 is a flowchart of a method for coupling from a fiber to aphotonic chip, according to some embodiments.

DETAILED DESCRIPTION

The following detailed description contains, for the purposes ofexplanation, numerous specific details in order to provide a thoroughunderstanding of the preferred embodiments of the disclosure. It isapparent, however, that the preferred embodiments may be practicedwithout these specific details or with an equivalent arrangement. Inother instances, some well-known structures and devices are shown inblock diagram form to avoid unnecessarily obscuring the preferredembodiments of the disclosure. The description should in no way belimited to the illustrative implementations, drawings, and techniquesillustrated below, including the exemplary designs and implementationsillustrated and described herein, but may be modified within the scopeof the appended claims along with their full scope of equivalents.

Described herein are apparatus and method for coupling light between anoptical fiber (or an optical fiber array) and a photonic chip. For thepurpose of this disclosure, a “channel” refers to the passage betweenone optical fiber and the photonic chip.

One way to achieve polarization diversity for a photonic chip is tosplit light from the output of the optical fiber into two orthogonalpolarization components by a polarization splitter (PS). The twocomponents are referred to as an X component and a Y component,respectively. One of the components, for example the X component, may becoupled into the photonic chip as a Transverse Electric (TE) mode, andthe other component, for example the Y component, may be coupled intothe photonic chip as a Transverse Magnetic (TM) mode. The PS can be usedwith a polarization rotator (PR), where one of the components can berotated by the PR so that both components can be in the same mode, be itTE or TM. The PS and PR can be referred to collectively as apolarization splitter-rotator (PSR). After performing desired operationsof the photonic chip separately for the two components, one componentcan be rotated again by another PR, and recombined with the othercomponent by a polarization combiner (PC). The PR and PC can be referredto collectively as polarization rotator-combiner (PRC).

An on-chip PS/PC or PSR/PRC can be physically large and may have largeoptical loss. An off-chip PS/PC or PSR/PRC can have lower optical loss,by using bulk materials and thin films. However, the cost of off-chipPS/PC or PSR/PRC can be high unless they are shared between manychannels.

On-chip PSR/PRC may be used with edge couplers. However, edge couplersmay not scale to a large channel count because of the comparativelylarge outer diameters of optical fibers. For example, a single line of1000 single mode optical fibers can be 12.5 cm wide, which can be 5times the width of the edge of a large size chip.

A surface grating coupler (SGC) is an optical device that may befabricated on a photonic chip, which couples light from an incidentoptical beam into one or more waveguides of the photonic chip. The SGCmay also be used in the opposite direction to couple light from thephotonic chip to an optical fiber. An SGC may couple light into a TEpolarization mode of a waveguide. Such an SGC is called a TE SGC.Another type of an SGC may couple light into a TM polarization mode of awaveguide. Such an SGC is called a TM SGC. Both TE SGC and TM SGC areconfigured for operation only in a single polarization mode. For lightof a given wavelength and incident angle, a TE SGC and TM SGC havedifferent structures, because the effective refractive index of TE modesand TM modes are different. SGCs include very small scale features thatare challenging to make in low-cost manufacturing processes, and it isdifficult to achieve the nominal coupling angle with high yield. Thecoupling angle of the TE SGC and TM SGC react differently to variationsof the manufacturing process. Thus, it is extremely challenging tomanufacture TE SGC and TM SGC of the same coupling angle in the samephotonic chip, with high yield. Accordingly, it may be advantageous toconstruct a dual-polarization coupling apparatus having only one type ofSGC.

A SGC that couples incident light of both polarization components torespectively TE and TM modes of one or two waveguides is known as adual-polarization SGC. However, dual-polarization SGCs are much moredifficult to manufacture than single-polarization SGCs, and the opticalloss of a dual-polarization SGC is several dB worse than that of asingle-polarization SGC, be it TE SGC or TM SGC. Thus, it may beadvantageous to construct a dual-polarization coupling apparatus,without using any dual-polarization SGC.

In accordance with various embodiments of the disclosure, an opticalcoupling apparatus is provided incorporating an off-chip PS/PC andsingle-polarization SGCs.

FIG. 1 is a simplified illustration of an optical coupling apparatus100, according to one embodiment of the disclosure.

Referring to FIG. 1, light beam 103 from an optical fiber 102 iscollimated by a collimating microlens 104. A PS 106, referred to as apolarization splitting beam displacer, splits the light beam 103collimated by the collimating microlens 104 into orthogonally polarizedX, Y component beams 105, 107. In the embodiment illustrated by way ofFIG. 1, the polarization splitting beam displacer 106 is a birefringentplate. As will be explained in more detail, the polarization splittingbeam displacer 106 includes polarization-dependent properties that cancause the X, Y component beams 105, 107 to propagate along differentoptical paths. In other words, the polarization splitting beam displacer106 splits a dual-polarization light beam 103 into twosingle-polarization component beams 105, 107, which are spatiallyseparated from each other. The separation may be achieved at the outputof the polarization splitting beam displacer 106. Alternatively oradditionally, the X, Y component beams 105, 107 can exit thepolarization splitting beam displacer 106 at an angle with respect toeach other. The X, Y component beams 105, 107 are passed through afocusing microlens 108 which directs the X and Y component beams 105,107 separately onto a photonic chip 112. The photonic chip 112 includesfirst and second SGCs 110 a, 110 b for coupling the X and Y componentbeams 105, 107, respectively, to the photonic chip 112. The first andsecond SGCs 110 a, 110 b are single polarization SGCs configured foroperation in a same polarization state (such as TE, for example). Aswill be explained in more detail, the X and Y component beams 105, 107are directed away from each other by the focusing microlens 108 andtowards the photonic chip 112, such that angles of incidence of the Xand Y component beams 105, 107 onto the photonic chip 112 correspond tocoupling angles of the corresponding SGCs 110 a, 110 b.

The first and second SGCs 110 a, 110 b are orthogonally disposed on thephotonic chip 112. That is, the first and second SGCs 110 a, 110 b havelayout axes forming a 90° angle with respect to each other. Inparticular, the SGC layout axis 116 (i.e., parallel to the y directionin FIG. 1) of the SGC 110 a for X component is orthogonal to the SGClayout axis 114 (i.e., parallel to the x direction in FIG. 1) of the SGC110 b for Y component. This way, the electric field 124 of the Xcomponent beam 105 can be orthogonal to the SGC layout axis 116 of thefirst SGC 110 a. As well, electric field 126 of the Y component beam 107can be orthogonal to the SGC layout axis 114 of the second SGC 110 b.Thus, both SGCs 110 a, 110 b can operate in a same polarization state.In this particular example, both SGCs 110 a, 110 b have TE polarizationeven though the X and Y component beams 105, 107 have orthogonalpolarizations. By using the optical coupling apparatus 100, light frompolarization-uncontrolled optical fiber(s) 102 can be coupled to twicethe number of polarization-defined waveguides, using the SGCs 110 a, 110b.

FIG. 2 is an illustration of the optical paths of the X, Y componentbeams 105, 107 in the optical coupling apparatus 100. As shown in FIG.2, the central rays of X and Y component beams 105, 107 propagatethrough the focusing microlens 108 with intersections 118 a, 118 brespectively and do not propagate on the optical axis of the focusingmicrolens 108. To simplify the Figures, the microlenses 104, 108 areillustrated as flat structures. In practice, microlenses 104, 108 mayhave a 3-dimensional profile.

The optical axis of a microlens in an array of microlenses may bedefined as a line normal to the plane of the microlens array thatcontains the microlens, such that an optical ray propagating along theoptical axis passes through the microlens without any angular deviation.For example, if the microlens is of refractive type and has a planesurface and a spherical surface, then the optical axis is through themost prominent point of the spherical surface. The optical axis of amicrolens may not be at its physical center.

If the center of a beam passes through the optical axis of a microlens,then the beam generally does not acquire any angular deviation. A beamwhose central ray passes through a microlens off the optical axisacquires an angular deviation. This known physical principle isexploited to manipulate the light to achieve different angles for the Xcomponent beam 105 and the Y component beam 107.

The central rays of X and Y component beams 105, 107 can impinge on thefocusing microlens 108 at different points on the focusing microlens 108and/or at different angles with respect to its optical axis. Thefocusing microlens 108 can be arranged such that at least one componentbeam 105, 107 is an off-axis ray (i.e., a ray that propagates in a planeincluding the optical axis of the microlens but not along the opticalaxis). For example, at least one of the X and Y component beams 105, 107can impinge on the focusing microlens 108 parallel to its optical axisbut offset with respect to its optical axis. Alternatively, the focusingmicrolens 108 can be arranged such that at least one component beam 105,107 is a skew ray (skew beam) (i.e., a ray that is neither parallel tonor intersecting the optical axis of the microlens). The focusingmicrolens 108 is arranged such that the X component beam 105 is directedalong +y direction, and the Y component beam 107 is directed along +xdirection.

The coupling angle of the SGC 110 a, 110 b and the propagation angle inthe polarization splitting beam displacer 106 may be different.Therefore, the focusing microlens 108 can be used to change the anglesof the component beams 105, 107 so that they match the coupling angle ofthe SGC 110 a, 110 b. However, it is to be understood that the focusingmicrolens 108 may deflect just one of the component beams 105, 107. Inother words, one of the component beams 105, 107 can pass through thefocusing microlens 108 without acquiring any angular deviation. Thefocusing microlens 108 focuses each component beam 105, 107 into a smallspot that matches the spot size of the SGC 110 a, 110 b.

For purposes of simplicity, FIGS. 1 and 2 only illustrate the light beamfrom one optical fiber 102. It should be appreciated that the opticalfiber 102 can be part of an optical fiber array and a similar result canbe replicated across the fiber array. The optical fibers may be arrangedin a line, or as a two-dimensional array. For the purpose of thisdisclosure, the term “array” is used to refer to a one-dimensional arrayor a two-dimensional array. The optical fiber(s) may be arranged for thelight to enter the polarization splitting beam displacer 106 at anydesired angle. The optical fiber array may be on a square grid,rectangular grid, hexagonal grid, or another grid. The grid may beuniform or non-uniform.

Although many embodiments are explained with reference to TE SGCs, itshould be appreciated that TM SGCs can be used instead of TE SGCs. Forexample, the SGCs can be all TM SGCs followed by TM circuits in thephotonic chip 112, or the SGCs can be all TM SGCs followed by PRs to TEcircuits in the photonic chip 112. The orthogonality of the SGC layoutaxes 116, 114 of the first and second SGCs 110 a, 110 b enables the useof single-polarization SGCs for dual-polarization coupling purposes.

FIG. 3 is a perspective view of an optical coupling apparatus 200,according to another embodiment of the disclosure. Similar to theembodiment of FIG. 1, the light 103 from the fiber 102 becomes separatedfor X, Y component beams 105, 107 after passing through the polarizationsplitting beam displacer 106. However, in the embodiment illustrated byway of FIG. 3, the X component beam 105 propagates through a firstfocusing microlens 108 a for directing the X component beam 105 to thefirst SGC 110 a for X component. The Y component beam 107 propagatesthrough a second focusing microlens 108 b separate from the firstfocusing microlens 108 a for directing the Y component beam 107 to thesecond SGC 110 b for Y component.

FIG. 4 is an illustration of the optical paths of the X, Y componentbeams 105, 107 in the optical coupling apparatus 200. Similar to theembodiment of FIG. 1, the central ray of X component beam 105 does notpropagate through the focusing microlens 108 a via its optical axis(with intersections 118 a). As well, the central ray of Y component beam107 does not propagate through the focusing microlens 108 b via itsoptical axis (with intersection 118 b). The focusing microlenses 108 a,108 b can be arranged such that at least one component beam 105, 107 isan off-axis ray. For example, at least one of the X and Y componentbeams 105, 107 can impinge on the corresponding focusing microlenses 108a, 108 b parallel to their respective optical axis but offset withrespect to their respective optical axis. Alternatively, the focusingmicrolenses 108 a, 108 b can be arranged such that at least onecomponent beam 105, 107 is a skew ray (skew beam). The focusingmicrolens 108 a is arranged such that the X component beam 105 isdirected along +y direction, and the focusing microlens 108 b isarranged such that Y component beam 107 is directed along +x direction.

As explained with reference to FIGS. 1 and 2, the focusing microlenses108 a, 108 b may be used to change the angles of the component beams105, 107. However, it is to be understood that the focusing microlenses108 a, 108 b may deflect only one or none of the component beams 105,107. In other words, one or both component beams 105, 107 can passthrough the focusing microlenses 108 a, 108 b without acquiring anyangular deviation. The focusing microlenses 108 a, 108 b focus eachcomponent beam 105, 107 into a small spot that matches the spot size ofthe SGC 110 a, 110 b.

FIG. 5 is a perspective view of the optical coupling apparatus 200including a two-dimensional optical fiber array 202. As shown, thetwo-dimensional optical fiber array 202 is a 2×2 array, and X, Ycomponent beams 105, 107 from one optical fiber of the 2×2 array aretraced through the optical coupling apparatus 200.

When the optical fiber 102 is a member of an optical fiber array 202,the collimating microlens 104 is a member of a collimating microlensarray 204 and the focusing microlens 108 is a member of a focusingmicrolens array 208. The polarization splitting beam displacer 106 canbe used to separate light collimated by each microlens of thecollimating microlens array 104 into orthogonally polarized X and Ycomponent beams. The SGCs on the photonic chip 112 form a SGC array 210including first and second SGC sub-arrays 210 a, 210 b configured forcoupling the X and Y component beams, respectively, to the photonic chip112. As discussed above, the SGCs in the SGC array 210 are configuredfor operation in a same polarization state (such as TE). Every SGC for Xcomponent in the first SGC sub-array 210 a is orthogonally disposed onthe photonic chip 112 with respect to a SGC for Y component in thesecond SGC sub-array 210 b. The dashed lines in FIG. 5 are drawingconstruction lines, to illustrate the relative position of the elements.Although in the embodiment illustrated in FIG. 5, a single polarizationsplitting beam displacer 106 is provided to split all the beams from thefiber array 202, it should be understood more than one polarizationsplitting beam displacer may be used in other implementations. Thecollimating 204 and focusing 208 microlens arrays may include manymicrolenses for collimating light emitted by multiple optical fibers inan optical fiber array.

FIG. 6 is an example of a two-dimensional optical fiber array 202 thatcan be used as part of or in connection with the optical couplingapparatus 100, 200. It is to be understood that the figure is providedonly for illustration of a particular example and other arrangements ofthe two-dimensional optical fiber array are possible.

According to various embodiments, the polarization splitting beamdisplacer 106 has polarization-dependent properties enabling splittingof an incident dual-polarization light into two orthogonal components. Atypical polarization splitting beam displacer uses a crystal that has arefractive index which varies depending on the polarization and thedirection of propagation of light. Materials for polarization splittingbeam displacers may include Yttrium Vanadate (YVO₄) crystal, BariumBorate (α-BBO) crystal, Calcite crystal, Rutile (TiO₂) crystal, and thelike. It should be understood that various other suitable materials canbe used for the polarization splitting beam displacer 106. Thepolarization splitting beam displacer 106 may be mated to the microlensarrays (the collimating microlens array 204 and/or the focusingmicrolens array 208) using an optically transparent adhesive. With aYVO₄ crystal, a polarization splitting beam displacer can produce 0.1 mmlateral displacement for each 1.0 mm thickness of crystal. Thus, adisplacement of 127 μm may be achieved with a crystal of approximately1.3 mm thickness (1:10 ratio). Crystals can be precision polished toachieve accurate beam displacement.

Depending on the orientation of the incident light with respect to theaxis of the crystal, the polarization splitting beam displacer 106 maypass one polarization component undeviated and the other at an angle, ormay deviate both component beams 105, 107 by equal and opposite angleswith respect to the incident light 103, or may deviate both componentbeams by non-equal angles. In all cases, the purpose is to create twocomponent beams 105, 107 which propagate differently through thepolarization splitting beam displacer 106, so as to produce a separationat the output of the polarization splitting beam displacer. The beams105, 107 exiting the polarization splitting beam displacer 106 may beparallel to each other, and/or parallel to the incident beam 103, or maybe angled with respect to each other and/or the incident beam. All ofthe above implementations are possible, and depending on the desiredcharacteristics any suitable arrangement and design can be used.

Various types of microlenses, such as refractive, diffractive,spherical, aspherical, elliptical, or graded-index (GRIN) microlenses,may be used for the collimating microlens 104 and/or focusing microlens108. As well, microlenses with various manufacturing materials andmethods may be used including glass microlenses, polymer microlenses,etched microlenses, deposited microlenses, or diffusion microlenses. Theparticular design and manufacture of the microlenses can be selectedbased on cost and specification requirements of a particularimplementation.

In some embodiments, one or both of the collimating and focusingmicrolenses 104, 108 may comprise compound lens elements to reduce anundesired optical aberration such as spherical aberration, chromaticaberration and/or coma aberration.

Mechanical spacers may be provided between successive elements toachieve correct optical beam focusing distance(s). In particular,mechanical spacers may be placed between the fiber 102 and thecollimating microlens 104, between the collimating microlens 104 and thefocusing microlens 108, and/or between the focusing microlens 108 andthe photonic chip 112. The mechanical spacers are used for achievingcorrect optical distances between the elements. A spacer may be provideddefining a distance between the focusing microlens 108 and the photonicchip 112. Each mechanical spacer may be a planar optical element, withoptional mechanical structures. The part of each mechanical spacerthrough which the beam passes may be made of air, glass, index-matchingadhesive, or any other suitable transparent optical material, dependenton factors such as the type of the microlenses.

According to the disclosure, for cases where the microlenses 104, 108are refractive, diffractive or GRIN type, the gaps between the fiber 102and the collimating microlens 104 and between the focusing microlens 108and the photonic chip 112 may be filled with air or other medium of lowrefractive index. When the microlenses 104, 108 are GRIN microlenses,these gaps may be filled with index matching material such as opticaladhesive. Anti-reflection coatings may be used to reduceback-reflection.

According to the embodiment illustrated by way of example by FIG. 3, forlight beam from one fiber 102, two focusing microlenses 108 a, 108 b areused for directing the X and Y component beams, respectively, onto thefirst and second SGCs 110 a, 110 b. FIGS. 7 and 8 illustrate a spatialrelationship of the focusing microlenses 108 a, 108 b, according to suchan embodiment.

FIG. 7 is a cross-sectional view of a collimating microlens array 204;and FIG. 8 is a cross-sectional view of a corresponding focusingmicrolens array 208. The collimating microlenses highlighted byrectangle 214 in FIG. 7 may be of the same scale as the focusingmicrolenses highlighted by rectangle 216 a, 216 b in FIG. 8. As shown,the focusing microlens array 108 can include first and second focusingmicrolens sub-arrays 208 a, 208 b for focusing the X and Y componentbeams, respectively, onto the first and second SGC sub-arrays 210 a, 210b. The first focusing microlens sub-array 208 a for focusing the Xcomponent beams is symbolized by circles with X; and the second focusingmicrolens sub-array 208 b for focusing the Y component beams issymbolized by circles with Y. Focusing microlenses highlighted byrectangle 216 a in FIG. 8 are used for focusing the X component beamsfrom the collimating microlenses highlighted by rectangle 214 in FIG. 7.Focusing microlenses highlighted by rectangle 216 b are used forfocusing the corresponding Y component beams. As illustrated in FIG. 8,the first and second focusing microlens sub-arrays 208 a, 208 b arearranged in alternating rows in the focusing microlens array 208. Thealternating rows can form a non-rectangular pattern of microlenses. Inone particular embodiment, a focusing microlens 108 b for Y componentcan be offset laterally with respect to a corresponding focusingmicrolens 108 a for X component by an amount generally equal to theradius of the focusing microlens. In such an example, the focusingmicrolens 108 b for Y component is arranged diagonally at 60 degreesfrom the corresponding focusing microlens 108 a for X component toachieve a nearly hexagonal close-packed arrangement (illustrated byhexagon 218). It should be understood that other suitable spatialrelationships between the X focusing microlenses 108 a and Y focusingmicrolenses 108 b can be arranged.

In the embodiment illustrated in FIG. 7 and FIG. 8, the number offocusing microlenses in the focusing microlens array 208 is twice thenumber of collimating microlenses in the collimating microlens array204. In some implementations, microlenses in the focusing microlensarray 208 may be closer to each other than those in the collimatingmicrolens array 204, because the focusing microlens array 208 has twicesmaller pitch as the collimating microlens array 204. Thus, there may bemore clipping of the edges of the beams at the collimating microlensarray 208.

According to an alternative embodiment, an optical coupling apparatus isprovided where two beams can share a microlens of the focusing microlensarray 208. The two beams may be an X polarization beam from one opticalfiber and a Y polarization beam from a different optical fiber.Accordingly, the focusing microlens array 208 may not require a secondsubarray of microlenses. Instead, the focusing microlens array 208 canhave additional row(s) and/or columns(s) at the edge(s) of the array.The center of each of the two beams that share a particular focusingmicrolens 108 may impinge on the shared microlens at a different angleand/or at a different point on the microlens. Thus, the microlens candirect them to different SGCs 110 a, 110 b at their respective desiredangles, similar to the embodiments described above.

FIG. 9 illustrates an SGC array 210 on the photonic chip 112 withpolarization diversity, according to one embodiment of the disclosure.As shown in the figure, the SGC array 210 includes a first SGC sub-array210 a for coupling the X component beams and a separate second SGCsub-array 210 b for coupling the Y component beams. The first SGCsub-array 210 a receives the X component beams and is connected to aphotonic circuit 120 a for processing X component. The second SGCsub-array 210 b receives the Y component beams and is connected to aphotonic circuit 120 b for processing Y component. In this particularembodiment, the SGCs of the sub-arrays 210 a, 210 b are shown as havingthe same physical structure, but rotated 90 degrees from each other.Optical waveguides for the X, Y component beams implement re-entrantrouting to avoid crossing between the waveguides 122 a for X componentbeams and the waveguides 122 b for Y component beams. This may berealized for a large fiber count such as 1000 fibers.

FIG. 10 is a close-up view of two surface grating couplers 110 a, 110 bof the SGC array 210 of FIG. 9. The two SGCs 110 a, 110 b are bothsingle polarization such as TE polarization. The X component beam 105 isin a plane that is formed by the normal to the photonic chip 112 and anoutput waveguide 122 a of the SGC 110 a. The Y component beam 107 is ina plane that is formed by the normal to the photonic chip 112 and anoutput waveguide 122 b of the second SGC 110 b.

A dotted circle 126 a illustrates a cross-section of the X polarizationcomponent beam 105 at a distance above the photonic chip 112. Thedirection of propagation is mostly into the page (along the z axis) andat an acute angle to the y direction. A cross-section of the Xpolarization component beam 105 at a surface of the photonic chip 112 isillustrated by a dotted circle 128 a. The X polarization component beam105 propagates through an output waveguide 122 a in TEO mode containingthe X component. The electric field 124 a of the X component beam 105 inthe waveguide 122 a is along the x direction in the plane of thephotonic chip 112.

A dotted circle 126 b illustrates a cross-section of the Y polarizationcomponent beam 107 at a distance above the photonic chip 112. Thedirection of propagation is mostly into the page (along the z axis) andat an acute angle to the y direction. A cross-section of the Ypolarization component beam 107 at the surface of the photonic chip 112is illustrated by the dotted circle 128 b. The Y polarization componentbeam 107 propagates through an output waveguide 122 b where TEO mode isexcited, containing the Y component. The electric field 124 b of the Ycomponent beam 107 in the waveguide 122 b is along the y direction inthe plane of the photonic chip 112.

In this particular embodiment, the illustrated SGCs are of a curvedfocusing type. It should be understood that other suitable types ofgratings can be used, such as a straight grating followed by a taperthat narrows to an output waveguide.

In some embodiments, at least one of the X component beam 105 and Ycomponent beam 107 is off-axis when they intersect their respectivefocusing microlenses. This is to compensate for the angle induced by thepolarization splitting beam displacer 106 and direct the beamsseparately, so that each component beam 105, 107 can impinge on theirrespective SGC 110 a, 110 b at its corresponding coupling angle. Thecoupling angle of a SGC refers to the angle of incidence at which theinterference created by the incident beam hitting the SGC grating isconstructive and the corresponding SGC achieves a maximum efficiency. Inone particular embodiment, the coupling angle for the SGC 110 a forcoupling the X component beam 105 is the same as the coupling angle forthe SGC 110 b for coupling the Y component beam 107.

In some embodiments, the maximum efficiency may be achieved when theincident beam impinges on the SGC at a small angle to the vertical,typically in the range 10 to 20 degrees, to reduce back-reflection andfor easy manufacturing. However, in other embodiments, anormal-incidence SGC can be used where the SGC is of a type thatfunctions maximally with a normal incident beam. In such cases, thefocusing microlens(es) are arranged to produce beams that are normal tothe photonic chip 112.

FIG. 11 illustrates an example of intersections of X and Y componentbeams 105, 107 on their corresponding focusing microlenses 108 a, 108 b.As discussed above, at least one of the two orthogonal component beams105, 107 intersects the focusing microlens 108 a, 108 b off-axis.

As shown in FIG. 11, the microlens 108 a for X component can have itsoptical axis at 130 a, whereas the center of X component beam intersectsthe microlens at 132 a; and the microlens 108 b of Y component has itsoptical axis at 130 b, whereas the center of Y component beam intersectsthe microlens at 132 b.

It should be noted that the microlenses 108 a and 108 b are illustratedin FIG. 11 with arbitrary shapes, and it should be understood that, moregenerally, microlenses may be of any suitable shape. As described above,the optical axes of the microlenses may be in the physical centers ofthe microlenses, or may be offset from the physical centers. The opticalaxes of the microlenses may be within the microlenses or outside of therespective microlenses. As well, the X and Y microlenses may have thesame (or similar) or different parameters including shape, focal length,and the like. In one particular embodiment, the X and Y microlenses mayhave the same focal length and the lengths of the optical paths of the Xcomponent beam and the Y component beam can be generally the same.

Furthermore, the microlenses, and the focusing microlenses inparticular, may not have rotational symmetry, i.e. the surface of themicrolens may not be part of a surface that can be rotated withoutchanging its shape. One example of such a microlens is a microlens withastigmatism.

In various embodiments described above, the operation of the opticalcoupling apparatuses 100, 200 is illustrated with reference to lighttravelling in the direction from the optical fiber 102 to the photonicchip 112. For example, referring back to FIG. 1, the “collimating”microlens 104 collimates the light beam 103, and the “focusing”microlens 108 focuses the polarized X, Y component beams 105, 107.However, it should be understood that the optical coupling apparatuses100, 200 can be used or modified for coupling light travelling from thephotonic chip 112 to the fiber 102. The optical coupling apparatuses100, 200 can be further used or modified such that some optical beamscouple from the optical fibers to the photonic chip and other opticalbeams couple from the photonic chip to the optical fibers. Some opticalpaths may be bidirectional, coupling light in both directionssimultaneously. In a special case of the bi-directional scenario, the Xcomponent beam of one channel can be coupled from the optical fiber tothe photonic chip and the Y component beam of the channel can be coupledfrom the photonic chip to the optical fiber. This can be useful for asystem where an optical fiber is used for signals in both directions andpolarization can be a discriminator between forward and return signals.

When the optical coupling apparatus is used for coupling a photonic chip112 to an optical fiber 102 or optical fiber array 202, light may travelfrom the photonic chip to the optical fiber(s). In such cases, first andsecond SGCs 110 a, 110 b or SGC sub-arrays 210 a, 210 b are used foremitting X and Y component beams, respectively, from the photonic chip112. The focusing microlens 108 or focusing microlens array 208functions as a collimating microlens or collimating microlens array forcollimating the emitted X and Y component beams from the photonic chip112. The polarization splitting beam displacer 106 functions as apolarization beam combiner (or simply, a PC) for combining the X and Ycomponent beams collimated by the collimating microlens(es) into adual-polarization beam. The collimating microlens 104 or collimatingmicrolens array 204 functions as a focusing microlens or focusingmicrolens array. One skilled in the art would appreciate that a samemicrolens can be referred to as a collimating lens or a focusingmicrolens, dependent on the direction of light propagation. Similarly, apolarization splitting beam displacer can be referred to as apolarization beam combiner when the direction of the light propagationis reversed. It is therefore to be understood that the terms“collimating”, “focusing”, “splitter”, and “combiner” are merelyidentifiers and are not intended to imply direction of propagation oflight, which may be reversed as explained above. Furthermore, at smallbeam sizes, the micro-beams are never truly “collimated” in a sense thatthey are parallel, non-expanding optical beams. Rather, the beams arere-focused, with a corresponding transformation of Gaussian waist sizeand position.

FIG. 12 is an elevation view of the optical coupling apparatuses 100,200, according to various embodiments of the disclosure. For purposes ofsimplicity, FIG. 12 illustrates the Y component beam 107 undeviated inthe plane of the drawing and the X component beam 105 exiting thepolarization splitting beam displacer 106 at a displacement and an anglewith respect to the Y component beam 107. However as discussed above, itshould be understood that various other implementations are alsopossible. As well, the deviations of the X component beam 105 throughthe polarization splitting beam displacer 106 and the focusing microlens108 in FIG. 12 are illustrated as being in the plane of FIG. 12, itshould however be understood that the deviations of the X component beam105 may occur an arbitrary plane.

In some embodiments of the disclosure, the optical fibers 102 may bepositioned normal to the photonic chip 112, where light travels alongthe z axis, that is, perpendicular to the photonic chip 112, or at asmall angle to the z axis. According to such embodiments, the overallapparatuses 100, 200 may include long arrays of optical fibers 102,extending a large distance normal to the photonic chip 112 (i.e., alongthe z-axis).

In some embodiments, the polarization splitting beam displacer 106 caninclude or be used in combination with a reflective turning surface tochange the direction of the light propagation in order to reduce theheight of the overall apparatus. The polarization splitting beamdisplacer 106 can comprise a polarization-selective reflector. FIG. 13is an elevational view of an optical coupling apparatus 300, accordingto one such embodiment. In the embodiment illustrated by way of FIG. 13,the optical coupling apparatus 300 includes a polarization splittingbeam displacer 106′ which is provided with a reflective turning surface111 for reflecting the X component beam 105 and the Y component beam 107differently to achieve a spatial separation between the two componentbeams.

For example, the reflective turning surface 111 can be a mirror surfacesuch as a 45° turning mirror surface disposed in an optical path betweenthe collimating microlens 104 and the focusing microlens 108, as shown.The reflective turning surface 111 can include a total internalreflecting surface. The optical fibers 102 can thereby be arranged to beparallel or nearly parallel to the photonic chip 112. The overall heightcan thus be greatly reduced. The reflective turning surface 111 may be asurface of the polarization splitting beam displacer 106′ having areflective coating. Alternatively, the polarization splitting beamdisplacer 106′ may comprise two blocks joined with an angled surfacethat reflects the two polarization component beams at different angles,according to the difference in refractive index between twopolarizations. Many types of polarization splitting beam displacers canbe used that comprise birefringent elements, including for example,Glan-Taylor prism, Glan-Foucault prism, Glan-Thompson prism, Nicolprism, Nomarski prism, Wollaston prism, Senarmont prism, Rochon prism,etc. In addition to cube-type polarization splitting beam displacers,polarization-dependent thin films can be used as well. Suitable turningsurfaces can be used in combination with such polarization splittingbeam displacers to separate the X and Y polarization component beams.

Although all the elements in the coupling apparatus may be reciprocal inthat their behaviors are independent of the direction of lightpropagation, there may be spatial filtering and/or diffraction effectsthat can break the reciprocity when the elements are assembled in thecoupling apparatus.

For example, when an input light beam impinges on the SGC 110 a, 110 bin FIG. 12, the input light induces the desired mode and otherhigh-order modes in the photonic chip 112. An output light beam emittedfrom the SGC 110 a, 110 b is constructed from only the desired mode ofthe photonic chip 112, and thus the beam that it produces is notidentical to the incident beam. Further, the microlens can clip afraction of the light at the periphery of the beam. The opticalconfiguration may therefore include SGCs 110 a, 110 b and microlenses104, 108 that differ based on their intended direction of use. SGCs andmicrolenses may be manufactured using lithographic processes that permiteach element to be different, and hence such non-uniform opticalconfigurations can readily be manufactured. For example, for a channelwhere the light passes from the optical fiber 102 to the photonic chip112, the microlens on the collimating microlens array 204 may be largerthan the microlens on the focusing microlens array 208. For a channelwhere the light passes from the photonic chip 112 to the optical fiber102, the microlens on the collimating microlens array 204 may be smallerthan the microlens of the focusing microlens array 208. The method ofoptimizing the optical configuration may include analytical andnumerical electromagnetic modeling techniques such as ray tracing,paraxial Gaussian beam analysis, and three-dimensional finite differencetime domain calculations.

Most SGCs are configured for a spot size that matches the spot size ofthe optical fiber (e.g., about 10 μm diameter). However, the mostdifficult alignment step during manufacture is to align the beams at theSGCs. Thus, according to some embodiments of the disclosure, the spotsize at the SGC 110 a, 110 b may be made larger than the spot size atthe optical fiber 102. As a result, the alignment tolerance at the SGC110 a, 110 b can be relaxed. As an example, SGC designed forapproximately 25 μm beam diameter could be suitable for this purpose.The optical coupling apparatus can be arranged to provide such animaging magnification.

FIG. 14 illustrates an optical coupling apparatus 400 with imagemagnifying, according to an embodiment of the disclosure. In thisembodiment, FIG. 14 illustrates imaging of a Gaussian beam and about twotimes magnification of spot size at the SGC 110 a, 110 b compared to atthe optical fiber 102. That is, the spot size at the SGC 110 a, 110 b isabout two times the spot size at the optical fiber 202. The focal lengthof the microlens of the collimating microlens array 204 is f1 and thefocal length of the microlens of the focusing microlens array 208 is f2which equals approximately 2f1.

Referring to FIG. 14, the optical beam is shown by curves where theintensity of the light is 1/e²=14% of the intensity at the center of thebeam. The spot size, or radius of curvature of the beam is at a minimumvalue at one place along the beam axis, known as the beam waist. In FIG.14, the beam waist of the collimated beam is illustrated by 134, at theneck of the light in the polarization splitting beam displacer 106. Asillustrated in FIG. 14, the beam waist 136 at the SGC 110 a, 110 b islarger than the beam waist at the optical fiber 102. In this embodiment,the beam waist 136 at the SGC 110 a, 110 b is about two times the beamwaist at the optical fiber 102. Of course, the exact magnificationdepends on the required size of the beam waist 136 and may differ fromthe value of 2.

FIG. 15 illustrates an optical coupling apparatus 500 with imagemagnifying, according to an embodiment of the disclosure, showing aclipping effect.

The microlenses in the focusing microlens array 208 may be positionedclose to each other in order to make the apparatus small, which mayimpose limitations on the size of the focusing microlens 108. At thesame time, a desired beam should be sufficiently big to propagatethrough the polarization splitting beam displacer 106 as a collimatedbeam. Accordingly, the edge of the beam may be clipped by the edge ofthe focusing microlens 108.

The structure of the system in FIG. 15 is similar to that in FIG. 14with the same focal lengths f1, f2 as in FIG. 14. However, the diameterof the focusing microlens 108 is made smaller. As shown in FIG. 15, thebest position for the SGC 110 a, 110 b without clipping is at 109. Thisis closer than the best position for the SGC 110 a, 110 b with clipping.At this position, the beam waist 136 at the SGC 110 a, 110 b is morethan two times larger than the beam waist at the optical fiber 102.

Gaussian beam clipping is described in G. D. Gillen, C. M. Seck, and S.Guha, “Analytical beam propagation model for clipped focused-Gaussianbeams using vector diffraction theory,” Opt. Exp., vol. 18, no. 5, pp.4023-4040, 2010, incorporated herein by reference. General descriptionof Gaussian beams can be found in,http://www.rp-photonics.com/gaussian_beams.html, and R. Paschotta,“Gaussian beams” in the Encyclopedia of Laser Physics and Technology,first edition October 2008, Wiley-VCH, ISBN 978-3-527-40828-3, alsoincorporated herein by reference.

FIG. 16 illustrates a method for coupling between the optical fiber 102and the photonic chip 112, according to an embodiment of the disclosure.Light from the optical fiber 102 is collimated (1602) and separated(1604) into orthogonally polarized X and Y component beams 105, 107. Asdiscussed above, this can be done by a polarization splitting beamdisplacer 106, 106′ of any suitable structure and material to achieve aspatial separation between the X, Y component beams 105, 107. The X, Ycomponent beams 105, 107 are directed (1606) separately to the photonicchip 112, for example, either by a single focusing microlens 108, or bytwo separate focusing microlenses 108 a, 108 b (i.e., first and secondfocusing microlenses). The X, Y component beams 105, 107 are coupled(1608) to the photonic chip 112 by first and second SGCs 110 a, 110 b.As discussed above, the first and second SGCs 110 a, 110 b areorthogonally disposed on the photonic chip 112 and configured foroperation in a same polarization mode (such as TE). As also discussedabove, at least one of the X and Y component beams can be disposedoff-axis with respect to the optical axis of the corresponding focusingmicrolens. The X, Y component beams 105, 107 can thereby be directedseparately onto the photonic chip 112 at coupling angles of the firstand second SGCs 110 a, 110 b, respectively. A spot size of light fromthe optical fiber 102 can be magnified through the optical couplingapparatus.

According to various embodiments of the disclosure, a dual polarizationoptical coupling apparatus can be provided by assembling an opticalfiber array with off-chip PS/PC and single-polarization SGCs such as TESGCs. The single-polarization SGCs can be oriented such that X and Ycomponent beams each encounters only a single-polarization SGC. This isdone using a polarization splitting beam displacer and a set of (or twointerleaved sets of) off-axis microlenses to create a set of X componentbeams at one angle and a set of Y component beams at a different angle.The angles are aligned to the respective coupling angles of twointerleaved sets of SGCs where all grating couplers are of the samepolarization. The beams hit the microlenses off-axis laterally and/orvertically, so the coupling angles of X and Y gratings can be achieved.

According to one particular implementation, the collimated beam radiusmay be less than 25% of the diameter of the collimating/focusingmicrolens 104, 108, and has a Rayleigh range that is more than two timesthe thickness of the polarization splitting beam displacer 106. Theamount of optical power clipped by each microlens 104, 108 can be lessthan 1%. The microlenses of the collimating microlens array 204 may beon a rectangular grid of 127 μm by √{square root over (3)}×127 μm. Themicrolenses for X component (X microlenses) in the focusing microlensarray 208 can be arranged on the same grid. The microlenses for Ycomponent (Y microlenses) in the focusing microlens array 208 can be onthe same grid but offset laterally by 127 μm and at 60 degreesdiagonally from the X microlenses on a nearly hexagonal close-packedarrangement, as shown at 218 in FIG. 8. This provides a close packing ofthe focusing microlens array 208. Each beam can experience a two-timemagnifying telescope, as discussed above. The beam radius (or spot size)at the optical fiber 102 can be around 3 μm to 4 μm and the beam radius(or spot size) at the SGC 110 a, 110 b can be around 5 μm to 8 μm, whenthe focal length of the focusing microlens 108 is around twice the focallength of the collimating microlens 104. The alignment tolerance at theSGC 110 a, 110 b is proportional to the beam radius (or spot size) atthe SGC. Therefore, the alignment tolerance at the SGC 110 a, 110 b maybe increased by the magnification of the beam radius, and the apparatusmay be easier to assemble than an apparatus that lacks thismagnification.

By way of the optical coupling apparatus described, a large number ofoptical fibers can be coupled to a photonic chip with polarizationdiversity.

According to various embodiments, both polarization component beams canbe handled with the same polarization gratings. In theory, TE SGCs andTM SGCs can be made on the same photonic chip. However, the couplingangle reacts to manufacturing variations differently for TE SGCs and TMSGCs. Therefore, it may be difficult to yield both with the samecoupling angle. Further, the manufacturing process for the best TE SGCis not generally the same as for the best TM SGC, so the performance ofone or both will be compromised. Accordingly, a photonic chip that usesonly one SGC polarization will have a better performance and a higheryield than a chip that requires both SGC polarizations.

Off-chip PS/PC have been manufactured with high performance. By way ofthe dual polarization optical coupling apparatus provided according tovarious embodiments of the disclosure, the cost and size of the off-chipPS/PC can be shared across many channels, so the cost and size perchannel can be small. Further, the off-chip PS/PC is assembled withinthe optical coupling apparatus simply as a face-to-face alignment of twopolished surfaces, which can be achieved by an inexpensive assemblyprocess.

In principle, the dual polarization optical coupling apparatus accordingto the various embodiments of the disclosure may enable a low cost and alow loss assembly for a large fiber count on a photonic chip withpolarization diversity. Because it is grating coupled, re-entrantrouting can be used so the waveguides of X component do not need tocross the waveguides of Y component. A microlens imaging system canprovide a larger beam, e.g. two times bigger beam at the photonic chipthan at the optical fiber, reducing alignment tolerance by two times andalso reducing cost.

The optical coupling apparatus according to various embodiments may beused for reconfigurable fiber optic communications, particularly foroptical add/drop wavelength-division multiplexing (WDM) applications inmetro optical networks, WDM passive optical network (PON), wirelessaggregation network/Cloud-radio access network (C-RAN), and may be usedfor reconfigurable data center networks and high-performance computing,data center transceivers, data center core switching network, orcoherent optical transceivers in Metro and Long Haul.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. A method for coupling light between an opticalfiber and a photonic chip, comprising: collimating or re-focusing lightfrom the optical fiber; separating the light into orthogonally polarizedX and Y component beams; directing the X and Y component beams toseparate first and second locations, respectively, on the photonic chip;and coupling the X and Y component beams to the photonic chiprespectively by first and second surface grating couplers (SGCs)orthogonally disposed on the photonic chip at the first and secondlocations, respectively, and configured for coupling a same polarizationmode to the photonic chip.
 2. The method of claim 1, wherein directingthe X and Y component beams separately onto the photonic chip comprisesdirecting the X and Y component beams by first and second focusingmicrolenses, respectively, wherein at least one of the first and secondfocusing microlenses is disposed off-axis with respect to the X and Ycomponent beams.
 3. The method of claim 1, wherein coupling the X and Ycomponent beams to the photonic chip comprises coupling X and Ycomponent beams onto the photonic chip at coupling angles of the firstand second SGCs, respectively.
 4. The method of claim 1, furthercomprising magnifying a spot size of light from the optical fiber. 5.The method of claim 4, wherein a spot size of at least one of the X andY component beams at the photonic chip is larger than a spot size at theoptical fiber.
 6. The method of claim 1, wherein the coupling of lightbetween the optical fiber and the photonic chip is absent rotatingpolarization of any of the component beams.
 7. The method of claim 1,wherein the collimating or re-focusing comprises collimating, andwherein the separating is performed after the collimating.
 8. The methodof claim 1, wherein the optical fiber is disposed substantially parallelto the photonic chip, the method further comprising reflecting the lightexiting the optical fiber to propagate towards the photonic chip.
 9. Themethod of claim 1, wherein at least two of the collimating, theseparating, and the directing is performed by a same optical element.10. A method for coupling light between a photonic chip and an opticalfiber, comprising: emitting X and Y component beams by first and secondsurface grating couplers (SGCs), respectively, wherein the first andsecond SGCs are orthogonally disposed on the photonic chip at the firstand second locations and are configured for coupling a same polarizationmode from the photonic chip; combining the X and Y component beams intoa beam of light; and re-focusing the beam of light onto the opticalfiber.
 11. The method of claim 10, wherein the coupling of light betweenthe optical fiber and the photonic chip is absent rotating polarizationof any of the component beams.
 12. An apparatus for coupling lightbetween an optical fiber and a photonic chip, the apparatus comprising:a collimator for collimating or re-focusing light from the opticalfiber; a beam separator for separating the light into orthogonallypolarized X and Y component beams; a beam director for directing the Xand Y component beams to separate first and second locations,respectively, on the photonic chip, wherein the beam director isconfigured for coupling the X and Y component beams to the photonic chiprespectively by first and second surface grating couplers (SGCs)orthogonally disposed on the photonic chip at the first and secondlocations, respectively, and for coupling a same polarization mode tothe photonic chip.
 13. The apparatus of claim 12, wherein at least twoof the collimator, the beam separator, and the beam director comprise asame optical element.
 14. The apparatus of claim 12, wherein the opticalfiber, the collimator, and the beam separator comprise a firstsubassembly, and the beam director and the photonic chip comprise asecond subassembly.
 15. The apparatus of claim 12, wherein thecollimator comprises a microlens.
 16. The apparatus of claim 12, whereinthe collimator and the beam director comprise a microlens.
 17. Theapparatus of claim 12, wherein the beam separator comprises apolarization beam displacer.